Allow `summarize` to aggregate multiple benchmarks into one score

When measuring more than one benchmark, it would be nice to be able to aggregate the results into a single score. One common way to do this is to take the geometric mean of a set of results. This issue proposes adding a `--aggregate-benchmarks` flag to do exactly this. When enabled, `sightglass-cli summarize --aggregate-benchmarks` would emit a single, "aggregated by geomean" result for each of the phases using ``, e.g., for the benchmark column.